The present invention relates to an arrangement for controlling the effective pressure of a hydraulic braking fluid in a vehicle braking system in general, and more particularly to a controlling arrangement for use in a vehicle braking system provided with an auxiliary pressurized hydraulic fluid source and antiskid control equipment.
There are already known various constructions of control arrangements for use in vehicle braking systems, among them such which find particular use in vehicle braking systems provided with an auxiliary source of pressurized hydraulic fluid and antiskid control equipment. It is also known to provide the hydraulic braking system provided with a controlling arrangement of this type which is supplied with pressurized hydraulic fluid from the auxiliary pressure or energy source with a brake-pedal actuatable master cylinder device including at least one master piston and an associated replenishment chamber which is connected through a replenishment port with a space adjacent to an end surface of the master piston which faces toward the brake pedal, while the oppositely facing or working end surface of the master piston delimits a working compartment in the master cylinder device, and to supply the pressurized hydraulic fluid into the working compartment of the master cylinder device. One construction of a control arrangement of a hydraulic vehicle braking system of this type is known from the German, published patent application DE-OS No. 24 43 545, wherein the master cylinder device is constructed as a tandem master cylinder in which the master piston which is closer to the brake pedal is equipped with a piston rod which is integrally provided at its free end that is closer to the brake pedal with a circumferential flange, the outer diameter of the circumferential flange being greater than the outer diameter of the associated master piston. A travel limiting spring is interposed between a brake-pedal actuated brake valve arrangement which is disposed between the brake pedal and the tandem master cylinder and which controls the supply of the pressurized hydraulic fluid from the auxiliary energy source, on the other hand, and the circumferential flange, on the other hand. One of the main purposes of this travel limiting spring is to give the driver a good brake pedal "feel" during brake pedal actuation. During normal braking, that is, without antiskid control action, the pressurized hydraulic fluid from the auxiliary energy source is supplied through the brake valve of the brake valve arrangement to the replenishing chambers of the tandem master cylinder and from there to the secondary side of the master piston. The circumferential flange which has a greater diameter than the master piston and which is sealed at its area of circumferential juxtaposition with the housing of the master cylinder device assures that, during the normal operation, both master pistons are being held in their initial or starting position, while the circumferential flange abuts against an axial abutment which is rigid with the housing of the master cylinder device and which is disposed close to the brake pedal. The pressurized hydraulic fluid which is supplied to the secondary side of the master piston flows through axial filling bores in the master piston into the working compartments of the master cylinder device and from there to the associated wheel brake actuating cylinders under dynamically controlled conditions. In the event that the auxiliary energy source fails, the circumferential flange of the master piston which is closer to the brake pedal is no longer held in abutment with the axial abutment that is rigid with the housing so that the two master pistons can move in unison in the working direction, so that the master cylinder device then performs its usual function, that is, the master pistons pressurize the fluid present in the working compartments and thus control the operation of the wheel brake actuating cylinders in a static manner. As advantageous as this conventional arrangement may be in many respects, it also has an important disadvantage which resides in the rather complicated structure of the master piston which is closer to the brake pedal due to the provision of the enlarged circumferential flange at the end of the piston rod which is closer to the brake pedal and which has a diameter exceeding that of the master piston. Inasmuch as the master piston is arrested in its initial position during normal operation, there must be provided means in the form of a travel limiting spring for giving the driver a good brake pedal "feel" during the operation. The brake valve arrangement by itself is not suited for this purpose. Inasmuch as the master piston is stationary during normal operation (and thus does not perform any function), it cannot be recognized during the normal operation of the brake whether or not the master piston has defective regions (such as possible leakage location at the region of the lip sealing of the master piston).